Pancharatnam-Berry Metasurfaces Research Articles

Terahertz Multiple Beam Steering Using Graphene Pancharatnam-Berry Metasurfaces

Pancharatnam-Berry metasurfaces have the great advantage of manipulating electromagnetic wave. At the same time, graphene has attracted much attention due to its unique electromagnetic responses in different Fermi levels. Here, graphene-based Pancharatnam-Berry metasurfaces are numerically built to achieve beam steering in terahertz band. When graphene is in the low Fermi level 0.01 eV, eight meta-atoms are designed to form a gradient Pancharatnam-Berry phase. After Pancharatnam-Berry phase is formed, a dual-beam metasurface is constructed, and then a gradient metsurface is designed to produce anomalous reflection. Finally, convolution operation is used to design two metasurfaces, and they generate dual- and quad-beams under circular and linear polarizations, respectively. Therefore, the designed metasurfaces can generate multiple beams in space. When graphene is in the high Fermi level 0.50 eV, phase difference between eight meta-atoms disappears, and only mirror reflection exists. Meanwhile, there must be a middle value in Fermi level of graphene to get multiple beams. At this moment, there are anomalously reflected beams and mirror beam. Using different responses of graphene in different Fermi levels, beam steering is achieved in terahertz band. Our design may provide a method for beam steering and terahertz communication.

High-efficiency generation of far-field spin-polarized wavefronts via designer surface wave metasurfaces.

Achieving a pre-designed scattering pattern from an ultra-compact platform is highly desired for on-chip integration optics, but conventional techniques suffer from the limitations of bulky size, wavelength-scale modulation and low efficiency. Here, we propose a new strategy to efficiently generate arbitrary spin-polarized scattering far-field patterns from surface-wave (SW) excitations on a designer Pancharatnam-Berry (PB) metasurface. We find that a PB meta-atom serves as a subwavelength scatter to decouple impinging SW to a spin-polarized propagating wave (PW) with tailored amplitude and phase, and thus interference among PWs generated by scatterings at different PB meta-atoms can generate a tailored far-field pattern. As a proof of concept, we design and fabricate a series of PB metasurfaces in the microwave regime and experimentally demonstrate that they can generate desired radiation patterns within a broad frequency band, including unidirectional radiation, line/point focusing, vortex beam and hologram. These findings may stimulate important applications in on-chip integrated photonics.

Multifunctional phase modulated metasurface based on a thermally tunable InSb-based terahertz meta-atom

Terahertz (THz) metasurfaces composed of Pancharatnam-Berry (PB) meta-atoms have great potential for applications in THz imaging, biological sensing, and optical communication. However, traditional THz PB metasurfaces suffer from inflexible electromagnetic responses and complicated structures. Here, we propose a thermally tunable reflection-type InSb-based THz PB meta-atom, which can not only convert the incident circularly-polarized (CP) wave into cross-polarized components but also adjust the reflection efficiency by increasing the temperature of InSb from 220 K to 360 K. Moreover, various functional devices, including anomalous reflector, reflection-type metalens, and reflection-type OAM beam generators, are investigated with the finite difference time-domain (FDTD) method by using the proposed meta-atom. The working states of these devices can be switched from “ON” to “OFF” at the frequency of 1 THz successfully by changing the temperature of InSb from 220 K to 360 K. This work not only paves a way for the study of tunable multifunctional THz PB devices, but also promotes the practical applications of THz metasurfaces.

Reconstruction of multidimensional nonlinear polarization response of Pancharatnam-Berry metasurfaces

Pancharatnam-Berry (PB) metasurfaces have been considered as innovative optical devices for efficient manipulation of both the phase and the polarization of the electromagnetic field. Here, we present the simultaneous generation of circularly, elliptically, and linearly polarized states by the simplest PB metasurface with one-dimensional phase gradient unit cells. Furthermore, we propose that the reconstruction of a multidimensional nonlinear polarization response of a nanomaterial can be achieved in a single heterodyne measurement by active manipulation of the polarization states of incident light. Using multidimensional spectroscopy, we demonstrate the possibility to track both stationary and transient delocalized charge distributions via detecting plasmonic populations and coherences.

Fundamentals and applications of spin-decoupled Pancharatnam-Berry metasurfaces.

Manipulating circularly polarized (CP) electromagnetic (EM) waves at will is significantly important for a wide range of applications ranging from chiral-molecule manipulations to optical communication. However, conventional EM devices based on natural materials suffer from limited functionalities, bulky configurations, and low efficiencies. Recently, Pancharatnam-Berry (PB) phase metasurfaces have shown excellent capabilities in controlling CP waves in different frequency domains, thereby allowing for multi-functional PB meta-devices that integrate distinct functionalities into single and flat devices. Nevertheless, the PB phase has intrinsically opposite signs for two spins, resulting in locked and mirrored functionalities for right CP and left CP beams. Here we review the fundamentals and applications of spin-decoupled metasurfaces that release the spin-locked limitation of PB metasurfaces by combining the orientation-dependent PB phase and the dimension-dependent propagation phase. This provides a general and practical guideline toward realizing spin-decoupled functionalities with a single metasurface for orthogonal circular polarizations. Finally, we conclude this review with a short conclusion and personal outlook on the future directions of this rapidly growing research area, hoping to stimulate new research outputs that can be useful in future applications.

Ultrathin and high-efficiency Pancharatnam–Berry phase metalens for millimeter waves

Applying the Pancharatnam–Berry (PB) principle to half-wave plate (HWP) metasurfaces allows the manipulation of wavefronts along with the conversion of the handedness of circularly polarized incident waves by simply rotating the meta-atoms that compose the metasurface. PB metasurfaces (PBM) working in transmission mode with four or more layers have been demonstrated to reach levels of transmission efficiency near 100% but also have resulted in bulky structures. On the other hand, compact tri-layer ultrathin (λ/8) designs have reached levels near 90% but are more challenging than single- or bi-layer structures from a manufacturing viewpoint. Here, we propose a compact ultrathin (<λ/13) transmissive PBM with only two layers (which significantly simplifies the fabrication process) achieving a transmission efficiency level of around 90%, focusing the wavefront of a circularly polarized incident wave and converting its handedness. The metasurface is composed of identical bi-layered H-shaped unit cells (meta-atoms) whose transmission phases are chosen by introducing different rotation angles to each unit cell according to a lens spatial phase profile. The structure is analytically and numerically studied and experimentally measured, verifying an excellent behavior as an HWP PB metalens at 87 GHz.

High-performance and ultra-broadband vortex beam generation using a Pancharatnam–Berry metasurface with an H-shaped resonator

Vortex beams carrying orbital angular momentum (OAM) are expected to efficiently increase channel capacity in communication sectors, so ultra-broadband vortex beam generators with high performance are important in next-generation communication systems. Based on the concept of the Pancharatnam–Berry phase, we propose a metasurface with an H-shaped resonator for realizing vortex beam generation within an ultra-wideband frequency range from 7.3 to 21 GHz (relative bandwidth = 96.8%). The designed meta-atom can acquire high efficiency (efficiency ≥ 92%) conversion from the incident plane wave to the reflected vortex wave across the entire frequency band. To verify the performance of the proposed metasurface, a reflective array consisting of 18 × 18 single-layered elements with different rotated angles is simulated for generating the vortex beam with a topological charge of ±2. Through OAM spectral analysis, the left-handed components of the reflected electric fields and mode purity of the generated vortex beams under different frequencies are achieved and discussed in detail. Then the metasurface is fabricated and measured, and the numerical and experimental results coincide well, proving the effectiveness and high performance of the proposed design.

Switchable Metasurface with VO2 Thin Film at Visible Light by Changing Temperature

We numerically demonstrated switchable metasurfaces using a phase change material, VO2 by temperature change. The Pancharatnam–Berry metasurface was realized by using an array of Au nanorods on top of a thin VO2 film above an Au film, where the optical property of the VO2 film is switched from the insulator phase at low temperature to the metal phase at high temperature. At the optimal structure, polarization conversion efficiency of the normal incident light is about 75% at low temperature while that is less than 0.5% at high temperature in the visible region (λ∼ 700 nm). Various functionalities of switchable metasurfaces were demonstrated such as polarization conversion, beam steering, Fourier hologram, and Fresnel hologram. The thin-VO2-film-based switchable metasurface can be a good candidate for various switchable metasurface devices, for example, temperature dependent optical sensors, beamforming antennas, and display.

Magnetically active terahertz wavefront control and superchiral field in a magneto-optical Pancharatnam-Berry metasurface.

Nowadays, the manipulation of the chiral light field is highly desired to characterize chiral substances more effectively, since the chiral responses of most molecules are generally weak. Terahertz (THz) waves are related to the vibration-rotational energy levels of chiral molecules, so it is significant to actively control and enhance the chirality of THz field. Here, we propose a metal/magneto-optical (MO) hybrid Pancharatnam-Berry (PB) phase structure, which can serve as tunable broadband half-wave plate and control the conversion of THz chiral states with the highest efficiency of over 80%. Based on this active PB element, MO PB metasurfaces are proposed to manipulate THz chiral states as different behaviors: beam deflector and scanning, Bessel beam, and vortex beam. Due to the magnetic-tunablibity, these proposed MO PB metasurfaces can be turned from an "OFF" to "ON" state by changing the external magnetic field. We further investigate the near-field optical chirality and the chirality enhancement factors in far field of the chiral Bessel beam and vortex beam, achieving the superchiral field with the highest chiral enhancement factor of 40 for 0th Bessel beam. These active, high efficiency and broadband chiral PB metasurfaces have promising applications for manipulation the THz chiral light and chiroptical spectroscopic techniques.

High efficiency and broad bandwidth terahertz vortex beam generation based on ultra-thin transmission Pancharatnam–Berry metasurfaces* *Project supported by the National Natural Science Foundation of China (Grant No. 62071312), the Important R&D Projects of Shanxi Province, China (Grant No. 201803D121083), and the Shanxi Scholarship Council (Grant No. 2020-135).

The terahertz (THz) vortex beam generators are designed and theoretically investigated based on single-layer ultra-thin transmission metasurfaces. Noncontinuous phase changes of metasurfaces are obtained by utilizing Pancharatnam–Berry phase elements, which possess different rotation angles and are arranged on two concentric rings centered on the origin. The circularly polarized incident THz beam could be turned into a cross-polarization transmission wave, and the orbital angular momentum (OAM) varies in value by lℏ. The l values change from ± 1 to ± 5, and the maximal cross-polarization conversion efficiency that could be achieved is 23%, which nearly reaches the theoretical limit of a single-layer structure. The frequency range of the designed vortex generator is from 1.2 THz to 1.9 THz, and the generated THz vortex beam could keep a high fidelity in the operating bandwidth. The propagation behavior of the emerged THz vortex beam is analyzed in detail. Our work offers a novel way of designing ultra-thin and single-layer vortex beam generators, which have low process complexity, high conversion efficiency and broad bandwidth.

Broadband and high-efficiency ultrathin Pancharatnam-Berry metasurfaces for generating X-band orbital angular momentum beam

Vortex beams have been extensively investigated for their major applications in wireless communications. To obtain practical applications, vortex beam generators with high performance and simple structure are of vital importance. Here, we present a novel metasurface-based device to generate high-performance vortex beams. This device is a four-layer Pancharatnam-Berry metasurface with ultrathin thickness. The unit cell of each metasurface is composed of a cross dipole and a square ring resonator. The simulated and experimental results show that the proposed meta-device can convert a right-hand circularly polarized wave into a left-hand circularly polarized vortex beam in a broadband from 9.3 to 12 GHz. Moreover, the device has high transmission efficiency up to 79% and ultrathin thickness of 0.2λ (λ is the central frequency of operation frequency band).

Ultra-thin and high-efficiency full-space Pancharatnam-Berry metasurface.

Full-space metasurfaces (MSs) attract significant attention in the field of electromagnetic (EM) wave manipulation due to their advantages of functionality integration, spatial integration and wide applications in modern communication systems. However, almost all reported full-space metasurfaces are realized by multilayer dielectric cascaded structures, which not only has the disadvantages of high cost and complex fabrication but also is inconvenient to device integration. Thus, it is of great interest to achieve high-efficiency full-space metasurfaces through simple design and easy fabrication procedures. Here, we propose a full-space MS that can efficiently manipulate the circularly polarized (CP) waves in dual frequency bands by only using a single substrate layer, the reflection and transmission properties can be independently controlled by rotating the optimized meta-structures on the metasurface. Our full-space metasurface has the potential to design multifunctional devices. To prove the concept, we fabricate the device and measured it in microwave chamber. For the reflection mode, our metasurface can behave as a CP beam splitter at the frequency of f1 = 8.3 GHz and exhibit high efficiencies in the range of 84.1%-84.9%. For the transmission mode, our metasurface acts as a meta-lens at the frequency of f2 = 12.8 GHz for the LCP incidence, and the measured relative efficiency of the meta-lens reaches about 82.7%. Our findings provide an alternative way to design full-space metasurfaces and yield many applications in EM integration systems.

Revealing topological phase in Pancharatnam–Berry metasurfaces using mesoscopic electrodynamics

Abstract Relying on the local orientation of nanostructures, Pancharatnam–Berry metasurfaces are currently enabling a new generation of polarization-sensitive optical devices. A systematical mesoscopic description of topological metasurfaces is developed, providing a deeper understanding of the physical mechanisms leading to the polarization-dependent breaking of translational symmetry in contrast with propagation phase effects. These theoretical results, along with interferometric experiments contribute to the development of a solid analytical framework for arbitrary polarization-dependent metasurfaces.

A review of high-efficiency Pancharatnam–Berry metasurfaces

Manipulating circularly polarized (CP) electromagnetic waves as desired is important for a wide range of applications ranging from chiral-molecule manipulations to optical communication, but conventional natural-materials-based devices suffer from bulky configuration and low efficiencies. Recently, Pancharatnam–Berry (PB) metasurfaces have demonstrated strong capabilities to control CP waves in different frequency domains. In this article, we present a concise review on PB metasurfaces for CP light manipulations, focusing mainly on the research works done by our own group. After briefly introducing the working principles of PB metasurfaces, we separately discuss how to construct high-efficiency PB metasurfaces in reflection and transmission geometries, and how to utilize them to control CP waves in different frequency domains, including meta-lensing, meta-hologram, and surface couplers. Finally, we conclude this review with our perspectives on future developments of PB metasurfaces.

Low‐loss ultra‐wideband beam switching metasurface antenna in X‐band

This study proposes an ultra-wideband (UWB) metasurface-based beam-switching antenna system. A coplanar (CP) waveguide fed slot antenna (with 49% operating bandwidth) is coupled with a hexagonal metallic aperture to generate CP beam in the 10.2–10.8 GHz band. An octagonal split ring inclusion-based meta-element is designed to achieve 2π transmission phase variation with near-unity magnitude. The principle of the Pancharatnam–Berry metasurface is used to design an offset metasurface superstrate for tilting the main beam of the UWB antenna for the CP band. Measured results (S11, axial ratio, and radiation pattern) agree well with full-wave simulations. The fabricated X-band UWB aperture coupled antenna system uses the metasurface superstrate to achieve a broadside beam for the lower band and tilted beam for the upper band. This antenna system holds promise for next-generation vehicular and satellite communication applications.

High-efficiency electrically direction-controllable spoof surface plasmon polaritons coupler

We propose a design of high-efficiency and direction-controllable spoof surface plasmon polaritons (SSPPs) coupler based on a Pancharatnam–Berry (PB) metasurface and a diode-controlled, linear-to-circular polarization conversion metasurface (PCM). The PB metasurface was designed to achieve high-efficiency excitation of SSPPs by manipulating the phase distribution. The PCM was placed above the PB metasurface at a certain distance, and the propagation direction of SSPPs could be controlled by changing the bias voltage of PCM. To validate the feasibility of the proposed design, a SSPPs coupler was fabricated and assessed. The experimental results were in good agreement with the simulation results. The conversion efficiency from the free space wave to SSPPs was obtained to be as high as 0.76 at 10 GHz, and the propagation direction of SSPPs became controllable under the normal incidence of x-polarized waves on the PCM. Compared to conventional devices such as prism, grating, and gradient-index metasurfaces, the proposed SSPPs coupler is more suitable for SSPPs excitation, thereby providing an interesting route toward developing plasmonic devices.

Sorting full angular momentum states with Pancharatnam-Berry metasurfaces based on spiral transformation.

Full angular momentum states constitute a complete and higher state space of a photon, which are significant not only for fundamental study of light but also for practical applications utilizing cylindrical optics such as optical fibers. Here we propose and demonstrate a simple yet effective scheme of combining the spiral transformation with Pancharatnam-Berry (PB) metasurfaces for high-resolution sorting of full angular momentum states. The scheme is verified by successfully sorting full angular momentum states with 7 orbital angular momentum states and 2 spin angular momentum states via numerical simulations and experiments. We expect that our work paves the way for simple high-resolution sorting of full angular momentum states, which could be highly useful in both classical and quantum information systems.

Broadband and high-efficiency spin-polarized wave engineering with PB metasurfaces.

Manipulating circularly polarized (CP) light waves at will are highly important for photonic researches and applications. Recently, while Pancharatnam-Berry (PB) metasurfaces have shown unprecedented capabilities to control CP light, meta-devices constructed so far always suffer from the limitations of low-efficiency and narrow bandwidth. Here, we propose a scheme to construct PB metasurfaces with these two issues well addressed. To verify our idea, two PB meta-devices are designed and fabricated for achieving high-efficiency and broadband photonic spin Hall effect and focusing effect, respectively. Experimental results, in good agreement with full wave simulations, demonstrate the desired functionalities with efficiencies reaching 80% within an ultra-wide frequency band (8.2-17.3GHz). The proposed design scheme is generic and can be extended to high-frequency regimes. Our work can stimulate the realizations of high-performance and broadband PB meta-devices with diversified functionalities.

Photonic spin Hall effect based on broadband high-efficiency reflective metasurfaces.

Through the introduction of a broadband (∼1µm) high-efficiency (average above 90%) half-wave plate in the near-infrared (NIR) region, reflective Pancharatnam-Berry metasurfaces that can generate high-intensity (above 90%) left-circular polarization (LCP) and right-circular polarization (RCP) light for broadband (∼900nm) are designed. It provides a method to generate broadband high-efficiency circularly polarized (CP) light, which is a rare complex process in traditional chiroptical spectroscopy. To further show the broadband high-efficiency function of the meta-atom, a broadband high-efficiency reflective vortex beam generator based on metasurfaces is designed for RCP light in the NIR region. Moreover, with the appropriate arrangement, the metasurfaces are able to focus the vortex beam into a point to increase the intensity of the generated vortex beam.

Ultra-dispersive anomalous diffraction from Pancharatnam-Berry metasurfaces

Achieving ultra-dispersive diffractions is fundamentally important to improve the chromatic resolution of spectrometers for numerous applications, such as Raman measurements, atom and molecule identification, and so on. Gratings, as traditionally widely used diffraction elements, disperse chromatic light into different angles according to the phase matching condition, which resorts to the transverse reciprocal vectors of the grating lattice. Mathematically, gratings show higher diffraction dispersion for larger diffraction angles. Either increasing grating line frequencies or steepening the groove angles is adopted to enlarge the deflection angles. However, all of them cause problems of near-zero diffraction efficiency and complicated fabrication. Here, we realize an ultra-dispersive diffraction in the framework of metasurfaces using an alternative phase matching strategy, in which the contributions of both the reciprocal vectors of the lattice and the local wave vectors arisen by the phase gradient are considered simultaneously. The diffraction angle of more than 80° is achieved with the resulting dispersion 4 times larger than the Littrow grating counterparts.